Digital imaging devices for direct detection of X-rays and mass particles are based on semiconductor absorbers in which incident radiation is transformed into electron-hole pairs which can then be measured as an electrical signal by a readout unit. Besides superior sensitivity and spatial and temporal resolution compared to scintillator based indirect conversion, direct detectors offer spectral resolution, because the number of electron-hole pairs generated in the absorber is proportional to the energy of the incident particles and thus measurable by a pulse height analysis.
Imaging detectors, also called pixel sensors, employing direct conversion by means of semiconductor absorbers, can currently be implemented essentially in two different ways.
In the first, the absorber is bonded onto the readout chip in order to realize the connections needed to process the electrical signal from every absorber pixel. The commonly used bonding technique, used, for example, by the Medipix collaboration (http://medipix.web.cern.ch) or by Dectris AG (http://www.dectris.ch), is bump bonding. In this approach the absorber can in principle consist of any semiconductor material suitable for X-ray detection or particle detection, such as Si, Ge, GaAs and CdTe (see, for example, European Patent No. 0571135 to Collins et al., the entire disclosure of which is hereby incorporated by reference and relied upon).
The second implementation of direct imaging detectors is based on the monolithic integration of the absorber with the readout electronics. When employing standard silicon CMOS processing for the readout, such monolithic pixel sensors are based on Si absorbers. They are also called Monolithic Active Pixel Sensors (MAPS) and have been developed for charged particle tracking. Charge collection is enabled by n-implants in a lightly p-doped epitaxial layer typically 12-16 μm in thickness and occurs mainly by diffusion in the original design (see, for example, R. Turchetta et al., in Nucl. Instrum. Meth. Phys. Res. A 458, 677 (2001), the entire disclosure of which is hereby incorporated by reference and relied upon).
More recently, charge collection primarily by drift has been achieved, for example, within the LePIX project by means of epitaxial p−-layers with higher resistivities on the order of 400 Ωcm (see, for example, S. Mattiazzo et al. in Nucl. Instrum. Meth. Phys. Res. A 718, 288 (2013), the entire disclosure of which is hereby incorporated by reference and relied upon). Fully depleted monolithic pixel sensors have even shown potential for soft X-ray detection (see, for example, P. Giubilato et al., in Nucl. Instrum. Meth. Phys. Res. A 732, 91 (2013), the entire disclosure of which is hereby incorporated by reference and relied upon). Charge collection by drift not only reduces charge collection times from above a hundred nanoseconds to ten nanoseconds and less, but also greatly enhances collection efficiency and radiation tolerance (see, for example, W. Snoeys in Nucl. Instrum. Meth. Phys. Res. A 732, 91 (2013), the entire disclosure of which is hereby incorporated by reference and relied upon). On the other hand, charges are collected by drift only from a fully depleted epitaxial layer, which is limited both in thickness (typically below 30 μm due to epitaxy costs) and resistivity (at most between about 1-5 kΩcm). By contrast, bump bonded absorbers can easily have depleted regions a few 100 μm in width permitting charge collection from a much larger volume.
A path towards the realization of monolithic pixel sensors comprising thick, high resistivity absorber layers suitable for full depletion has recently been described by von Kanel in the International Patent Application PCT/IB2015/002385, the entire disclosure of which is hereby incorporated by reference and relied upon. The approach is based on recently developed low-temperature, covalent wafer bonding technique (see, for example, C. Flötgen et al. in ECS Transactions 64, 103 (2014), the entire disclosure of which is hereby incorporated by reference and relied upon). With temperatures typically below 300° C., this bonding technique is applicable to CMOS-processed readout wafers. The latter need, however, to be thinned to a thickness of about 10-20 μm before the covalent bonding step in order to permit depletion of the highly resistive bonded absorber wafer, at the possible expense of costs and yield.
There exists a need therefore for a simpler, cost-effective fabrication of monolithic pixel sensors with thick, fully depleted absorption layers offering enhanced radiation tolerance, speed and charge collection.